1. Field of the Invention
The present invention relates to a particle analyzing apparatus for imaging a particle suspended in a liquid. More specifically, the invention relates to a particle analyzing apparatus for analyzing a particle by introducing a sample liquid obtained by diluting blood, urine, or the like into a flow cell and imaging an individual particle such as a blood cell or a cell in urine.
2. Description of the Related Art
Among the conventional particle analyzing apparatuses of the above kind are an apparatus called a flow cytometer in which particles such as blood cells or other cells are introduced into a flow cell in an arranged manner and an individual particle is measured, and various particle analyzing apparatuses which image a particle such as a blood cell or some other cell and analyze the particle thus imaged.
An example of the flow cytometer is an imaging flow cytometer disclosed in Japanese Unexamined Patent Publication No. Hei. 5-142137 (U.S. Pat. No. 5,272,354) in which a still particle image is obtained by opening a shutter of an image intensifier having a high-speed gating function with continuous light illumination.
An example of the particle analyzing apparatus is a method and apparatus for analyzing cells in a liquid in which a particle detecting optical system is provided in addition to an imaging system, and a pulsed laser is caused to emit light with a predetermined delay after detection of a particle to obtain a still image of the particle as described in Japanese Unexamined Patent Publication No. Sho. 63-94156 (U.S. Pat. No. 4,786,165).
However, the above types of conventional particle analyzing apparatuses have a problem that where a flash lamp is used as a light source in imaging a particle such as a blood cell or some other cell in a flow cell, a clear image cannot be obtained when a sample liquid flows fast.
The light emitting period of a flash lamp is in the order of microseconds, i.e., it is relatively long. Therefore, when the light source having such a long light emitting period is used, a particle moves more than 5 .mu.m if the flow speed of a sample liquid is assumed to be 5 m/sec. Thus, a clear image cannot be obtained due to blurring.
That is, with flash lamp type light sources, it is difficult to attain a short light emitting period in the order of nanoseconds, which is required to prevent blurring from occurring in an image of a fast flowing particle. To obtain a clear image, it is necessary to use, for example, pulsed light sources such as a pulsed laser.
However, when a laser such as a pulsed laser is used, the spatial and temporal coherence of the laser may cause an interference fringe or diffraction, which also makes it difficult to obtain a clear image.
Examples of lasers include a pulsed laser and a CW (continuous wave) laser. A particle imaging system in which a CW laser, rather than a pulsed laser, is combined with a high-speed shutter (gate) also suffers from the problem of an interference fringe etc., and cannot produce a clear image.
As described above, pulsed lasers can realize a light emitting period on the nanosecond order or shorter. Further, lasers can improve the energy density per unit area to enable sufficient energy for imaging to be generated during a short light emitting period or a short gating period. However, in lasers, the spatial and temporal coherence causes an interference fringe, Fresnel diffraction or Fraunhofer diffraction, which deteriorates image quality.
When a sample liquid flows fast in a particle analyzing apparatus, it is necessary to use a light source such as a pulsed laser or a CW laser which can emit high-energy-density light to take a clear particle image. The spatial and temporal coherence is needed to obtain such a high energy density, which however deteriorates image quality.
A theoretical explanation as to why the spatial and temporal coherence causes various types of diffraction and an interference fringe will be made below starting with an analysis of problems we are facing now.
First, a description will be made of how an optical flow cell itself becomes a FP (Fabry-Perot) interferometer to cause an interference fringe.
Usually, the interference distance of a laser is defined by measuring a distance between two reflecting bodies. (An actual interference fringe is not considered to be caused by reflection by only two particular surfaces of an optical flow cell, but is instead caused by multiple reflection involving four surfaces in total.)
In theory, the interference distance of a laser is determined by the oscillation spectrum width. If the oscillation spectrum width is 30 GHz (corresponding to catalogue data of 1 cm.sup.-1 of an LD-pumped YAG laser), the difference between mean lifetimes of laser oscillation levels is roughly estimated from the above width such that EQU 1/(3.0.times.10.sup.10)=3.3.times.10.sup.-11 sec.
Further, the laser interference distance is calculated as EQU (3.0.times.10.sup.8 m/sec.).times.(3.3.times.10.sup.-11 sec.)=1.0.times.10.sup.-2 m
Where a flow cell has a shape of 4 mm.times.4 mm, an interference naturally occurs. In the case of an Ar laser, the laser interference distance is in the order of several kilometers. The narrow oscillation spectrum width is a measure of evaluating the temporal coherence of a laser. An interference fringe can be avoided by increasing the oscillation spectrum width.
Next, an explanation will be made of how Fresnel diffraction and Fraunhofer diffraction occur.
To cause laser oscillation, it is necessary that wavefront phases of respective excited atoms be identical (spatial coherence). The Fresnel diffraction is caused by a plane wave being diffracted by an end face of a cell or the like.
In the Fraunhofer diffraction, a ring-like fringe appears in a converged spot because of varying distances from respective points on the above-mentioned plane wave in the case where the plane wave is not one originating from a point light source but one further converged optically. Therefore, to improve the image quality by weakening the Fraunhofer diffraction, it is necessary to use an optical system which causes random spatial propagation.
According to the current quantum electronics theory and technique, the spatial coherence and the temporal coherence is interrelated and, therefore, it is impossible in a single laser light source to improve or deteriorate one of those.
The present invention has been made in view of the above circumstances, and has an object of providing, with the use of a laser light source for imaging, a particle analyzing apparatus which can obtain a high-quality particle image without being affected by any interference fringe or diffraction by illuminating a particle with a coherence-lowered laser beam.